1. Field of the Invention
This invention relates to a power unit, and more particularly to a portable AC power unit.
2. Prior Art
In recent years, portable AC power units have been increasing in number which are equipped with invertor devices for stabilizing the output frequency. For example, a portable power unit which generates AC power having a commercial frequency by means of an AC generator driven by an engine is known, e.g. by Japanese Provisional Utility Model Publication (Kokai) No. 59-132398, in which the engine is operated within a high speed range to cause the AC generator to generate high AC output, the output is converted into direct current, then the direct current is converted into alternating current by an invertor device, and the alternating current is outputted.
There is a demand for AC power units of this kind whose output waveform is as close to a sine wave as possible dependent upon their application. To this end, an attempt has been made to employ an invertor device of the pulse width-modulation type as the above-mentioned invertor device (Japanese Provisional Patent Publication (Kokai) No. 60-82098).
In engine generators of this kind, there have been used various kinds of protective means for protection of the output circuit. However, depending on the output characteristics and load characteristics of the generator, there is the possibility that the amount of load current outputted from the generator does not directly represent the state of load, which has spoiled the perfectness of conventional protection systems using the amount of load current as a direct indicator of the state of load. For example, in a protection system which is adapted to merely cut off power output from the engine generator when the load current has increased above a critical level, if a load device, such as an electric motor, is connected to the output circuit of the generator, and into which a large amount of electric current temporarily rushes when it is started, there is the possibility that power cut-off is carried out to a greater extent or more frequently than required, at the start of the load device. Therefore, such a system does not form the optimum protection system for output circuit protection.
Under these circumstances, the present assignee has already proposed, in Japanese Provisional Patent Publication (Kokai) No. 63-114527, a protection system for invertor-controlled type generators, which is capable of starting a load device such as an electric motor, by interrupting the power supply from the generator only over a predetermined time period when an overcurrent state in which the amount of load current exceeds a predetermined reference value is detected, and then resuming the power supply after the lapse of the predetermined time period, and thus repeating the interruption and resumption of the power supply.
In the proposed protection system, when the amount of load current exceeds the predetermined reference value, a protective circuit operates so that the engine generator per se, particularly an output-switching circuit (invertor circuit) thereof, etc. can be protected. However, from the viewpoint of protection of the load device, this does not provide proper protection.
For instance, when the output voltage is lowered due to a heavy load applied at the start of an electric motor and the like as the load device, or when the generator cannot generate rated output due to a short-circuit in a portion of the winding thereof, a transient current, which is abnormally too large for the load device, continues to flow until the load device starts to normally operate, over an intolerably long time period, eventually causing failure of the load device.